Field of the Invention
The present invention relates to a radiation generating tube, and a radiation generating apparatus including the radiation generating tube.
Description of the Related Art
Needs for small and lightweight medical modality with portability have been increasing with changing of social conditions such as improvement in the home medical care system and expansion of the range of treatment in emergency medical system. To respond to these needs, with developing of analysis diagnostic techniques in the medical field, various medical modalities have been developed. Radiation imaging apparatuses having a radiation source are, due to the sizes of the apparatuses, mainly fixed and installed in hospitals and medical testing facilities. Such radiation generating apparatuses having the radiation source are also required to be reduced in size and weight to use them as modalities applicable to home medical care and emergency medical care in disasters, accidents, and the like.
Factors determining the weight and size of radiation generating apparatuses include “radiation generation efficiency” and “shielding member”. The “radiation generation efficiency” means a radiation output intensity to kinetic energy of incident electrons, and a low conversion efficiency of the radiation generation efficiency has been a problem in size reduction and weight reduction. By increasing the radiation generation efficiency, size reduction and weight reduction of a drive circuit and a radiation member constituting a large part of a radiation generating apparatus in the volume and mass can be achieved.
The “shielding member” means heavy metallic parts disposed around the whole container of a radiation generation apparatus to prevent emission of radiation except for emission in a direction of necessary radiation flux. The shielding member is disposed to surround a radiation source, and therefore it increases the volume of the radiation generating member, which in turn increases the weight and size of the radiation generating apparatus.
As a method for increasing the “radiation generation efficiency”, a method of replacing a target from a reflection type target to a transmission type target has been proposed. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-545840 discusses a technique to increase “radiation generation efficiency” by a factor of 1.5 by replacing a conventional rotating-anode-type reflection-type target with a rotating-anode-type transmission-type target, and, in the same rotation conditions, to increase a peak of an electron injection amount by a factor of 1.3.
Further, in radiation generating apparatuses used for living body diagnosis in the medical field, a technique of providing, between the subject and a radiation source, a variable opening type collimator for determining a predetermined exposure range depending on a size of a specimen or a subject is known. In such a radiation generating apparatus, radiation emission to areas other than a predetermined observation field is not useful, and the variable opening type collimator is used to limit the amount of unnecessary exposure to the specimen or subject.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-545840 discusses a radiation generating apparatus having an electron source and a transmission type target separately arranged, in which, at the rear side of the transmission type target, that is, at the side opposite to the electron source of the transmission type target, a collimator for limiting an emission angle of the radiation generated at the transmission type target is provided.
Further, Japanese Patent Application Laid-Open No. 2010-115270 discusses a multiple radiation generating apparatus having a plurality of transmission type targets arranged in one-dimensional array or in two-dimensional array. In the multiple radiation generating apparatus disclosed in Japanese Patent Application Laid-Open No. 2010-115270, at the rear side of each of the radiation generating apparatuses, a forward shielding member for limiting an emission angle of the radiation generated at the transmission type target, and a variable opening type collimator for changing the emission direction and the emission angle of the generated radiation are arranged.
Further, an anode assembly having a silver target layer, a window material made of beryllium for supporting the target layer, and a window supporting member made of NiCuFe alloy is discussed in an article published by International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47, entitled “Improvements in low power, end-window, transmission-target X-ray tubes”. Further, the article published by International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47 entitled “Improvements in low power, end-window, transmission-target X-ray tubes” discusses that radiation due to the window supporting member, the radiation having quality different from that of the radiation due to the target layer contaminates the radiation spectrum. Accordingly, a collimator is disposed between a camera and a radiation generation tube to separate and detect the radiation due to the window supporting member and the radiation due to the target layer.
In the known radiation generating apparatuses having the reflection type target, by replacing the reflection type target with the transmission type target, in addition to the above-described increase in “radiation generation efficiency”, an advantage of “low output angle dependence in focal diameter” can be achieved.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-545840 discusses advantages of the transmission type target as compared to the reflection type target, that is, to “reduce apparent output angle dependence in focal diameter” on the target observed from the target side, and to “increase output angle” of the radiation flux. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-545840 further discusses a technique to cut a part of radiation flux having a large emission angle emitted from the transmission type target by the collimator disposed between the object and the radiation target.
As described above, an exposure range corresponding to an intended use can be provided by limiting the emission angle of the radiation flux generated from the radiation source having the transmission type target using the collimator. However, the emitted radiation flux may include radiation (referred to as off-focal radiation) generated at spots other than the focal spot of the electron beam formed on the target. The off-focal radiation is generated by irradiating with electrons a member disposed at a place other than the focal spot, the member including a heavy element. Such off-focal radiation decreases the resolution of the radiation diagnostic image, and consequently, the off-focal radiation is to be reduced as much as possible while maintaining the intensity of the radiation within the focal spot.
Hereinafter, with reference to FIGS. 7A to 7D, factors of the off-focal radiation generation will be described. FIGS. 7B to 7D are schematic views (upper diagram in each drawing) illustrating generation processes of off-focal radiation generated when an electron beam 5 is emitted toward a target 9 having a target layer 42 on one entire surface of a base member 41 for each track of backscattering reflected electrons, and distribution diagrams (lower diagram in each drawing) illustrating off-focal radiation generation distributions.
With reference to FIGS. 7A and 7B, off-focal radiation due to re-entering reflected electrons to target (hereinafter, referred to as off-focal radiation due to re-entering) will be described.
In FIG. 7A, the electron beam 5 directly enters the target layer 42 so that a focal diameter 30 is formed on the target layer 42 of the target 9. From the range of the focal diameter 30, a part of the directly entered electron beam 5 scatters backward, and a part of the backscattering electrons re-enters the target layer 42 due to a potential gradient existing between an electron emitting source (not illustrated) and the target layer 42, and becomes re-entering electrons 31.
The energy distribution of the backscattering electrons from the target layer 42 includes elastic scattering electrons. The elastic scattering electrons, according to the law of conservation of energy, re-enters the target layer 42 with the same energy as the electron beam 5 directly entering the target layer 42, and generates off-focal radiation 32 at the outside of radiation 51 within focal spot. In FIG. 7A, to facilitate the understanding, the radiation 51 within focal spot and the off-focal radiation 32 are expressed to emit radiation toward the front of the target 9 with emission angles. In actuality, however, although the radiation 51 and the radiation 32 have individual emission angle distributions respectively, the radiation 51 and the radiation 32 are emitted in all directions from the target layer 42.
FIG. 7B illustrates a generation mechanism of the off-focal radiation due to re-entering radiation and a distribution of the radiation generation area. In FIG. 7B, η(y) is a radiation intensity distribution due to re-entering reflected electrons to the target, and y shows a relative position of the target layer 42 in the in-plane direction. The radiation intensity distribution η(y) of the off-focal radiation due to re-entering radiation exceeds the focal diameter 30 due to direct incident electrons and is shown as a broad distribution 55. As described above, the off-focal radiation due to re-entering reflected electrons generates, at the outside of the focal spot due to the direct incident, off-focal radiation having a diameter larger than the focal diameter 30.
With reference to FIG. 7C, off-focal radiation due to target incidence reflected electrons to backward shielding member (hereinafter, referred to as off-focal radiation due to the backward shielding member) will be described. FIG. 7C illustrates a generation mechanism of the off-focal radiation due to the backward shielding member and a distribution of the radiation generation area.
FIG. 7C is similar to FIGS. 7A and 7B in that the electron beam 5 directly enters the target layer 42 to form the focal diameter 30 on the target layer 42 of the target 9. FIG. 7C differs from FIGS. 7A and 7B in that the arrangement includes a backward shielding member 40 located in a rearward position with respect to the target 9, that is, located at the side of the electron emitting source (not illustrated) as a peripheral structure of the target 9.
In FIG. 7C, from the range of the focal diameter 30 due to the directly entering electron beam 5, backscattering reflected electrons 33 are generated, and a part of the reflected electrons 33 enters the backward shielding member 40. The backward shielding member 40 is a member containing a heavy metal, and generates radiation in response to reception of the entering reflected electrons 33. A part of the generated radiation is emitted toward the front of the target 9. As a result, as illustrated in the lower diagram of FIG. 7C, an off-focal radiation intensity distribution ξ(y) is generated to have peaks at positions corresponding to the inner wall of the backward shielding member 40.
In FIG. 7C, ξ(y) is a radiation intensity distribution due to the target incidence reflected electrons to the backward shielding member, and y shows a relative position of the target layer 42 in the in-plane direction.
With reference to FIG. 7D, off-focal radiation due to target incidence of re-entering reflected electrons of target reflected electrons to the backward shielding member (hereinafter, referred to as off-focal radiation due to re-entering reflected electrons) will be described. FIG. 7D illustrates a generation mechanism of the off-focal radiation due to re-entering reflected electrons and a distribution of the radiation generation area.
FIG. 7D is similar to FIG. 7C in that the electron beam 5 directly enters the target layer 42 to form the focal diameter 30 on the target layer 42 of the target 9, and the arrangement includes the backward shielding member 40 located in the rearward position with respect to the target 9 as a peripheral structure of the target 9.
In FIG. 7D, from the range of the focal diameter 30 due to the directly entering electron beam 5, backscattering reflected electrons 33 are generated, and a part of the reflected electrons 33 enters the backward shielding member 40. Similar to the generation mechanism of the off-focal radiation due to backward shielding member, the backward shielding member 40 generates radiation in response to reception of the entering reflected electrons 33 and a part of the entered electrons elastically scatters. A part of the elastically scattering electrons (re-reflected electrons 34) re-enters the target layer 42. As a result, as illustrated in the lower diagram of FIG. 7D, a broad off-focal radiation intensity distribution ζ(y) is generated to have peaks at positions corresponding to the inside of the inner wall of the backward shielding member 40.
In FIG. 7D, ζ(y) is a radiation intensity distribution due to the target incidence of the re-reflected electrons of the target reflected electrons to the backward shielding member 40, and y is a relative position of the target layer 42 in the in-plane direction.
The individual radiation intensity distributions η(y), ξ(y), and ζ(y) are observed by a radiation detector (not illustrated) disposed in front of the target 9, that is, at the side of the base member 41 of the target 9.
The off-focal radiation to be solved in the present invention is generated due to at least one of the three types of off-focal radiation generated depending on a scattering angle θbs of the backscattering electrons of the target layer 42, an arrangement, and a material of the backward shielding member. The backscattering electrons have a continuous scattering angle probability distribution in the range of 0 degrees≦θbs<90 degrees, and consequently, normally, the three types of off-focal radiation are generated at the same time.
As described above, at least one embodiment of the present invention is directed to a radiation generating apparatus capable of reducing each of the off-focal radiation due to the three factors while maintaining the advantages of high power performance, and the small and lightweight properties of the radiation generating apparatus having the transmission target. Further, the present invention is directed to providing a radiation imaging apparatus having a radiation generating apparatus reducing the off-focal radiation and capable of obtaining a high-resolution shot image.
In the description of the above-described “off-focal radiation due to re-entering radiation” with reference to FIGS. 7A and 7B, to facilitate understanding, the backward shielding member 40 is not illustrated, however, in a target peripheral structure having the tubular backward shielding member 40, similarly to FIGS. 7C and 7D, the off-focal radiation due to re-entering radiation is generated.